docs: def has_constant_expr(expr):
      """
      Report whether a model expression has constant specific term.
      That is, a term explicitly specying whether the model should or
      should not include a constant. (e.g. '+ 1' or '- 1'.)
  
      Parameters
      ----------
      expr : str
          Model expression to check.
  
      Returns
      -------
      has_constant : bool
  
      """
      def has_constant(node):
          if node.type == 'ONE':
              return True
  
          for n in node.args:
              if has_constant(n):
                  return True
  
          return False
  
      return has_constant(patsy.parse_formula.parse_formula(expr))